Plasma generating device using an electromagnetic wave is being developed. For example, JP 2007-113570A discloses a plasma generating device that generates plasma discharge in an internal combustion engine by emitting microwave before or after the ignition of an air-fuel mixture. This plasma generating device allows creating local plasma using a discharge of ignition plug and this plasma can be enlarged by the microwave.
However, the conventional plasma generating device does not have enough plasma generation and enlargement efficiency against the consuming electric power. Further, the maintenance of plasma generation/enlargement efficiency is difficult to be maintained in high condition because the plasma state fluctuates due to variation of surrounding conditions.